Source Current.

Discussion in 'General Electronics Chat' started by Runs212, May 3, 2015.

  1. Runs212

    Thread Starter Member

    Mar 12, 2015
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    I'm trying to figure out why my mosfet is heating up. Please refer to the schematic. This is an inverter whose load is an LC resonant circuit. Based on my research, it seems my MOSFET is either not receiving enough current or receiving too much current. The datasheet states that the driver: IRS2103 sources up to 230mA. I have refered to the MOSFET's (FQA55N25) datasheet to see if it requires a more than 230mA but was unable to find answers.

    Here are a few things I had in mind to. I wanted to use an an optoisolator where i can source a much higher current from it's output, connnect that to my driver to enable it source a higher current to my mosfet. Could that help?

    I'd appreciate your feedback.

    20150502_205404s.jpg
     
  2. k7elp60

    Senior Member

    Nov 4, 2008
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    What is the frequency of the square wave that you are supplying to the gates of the MOSFET's, and what is the value of the componets in your series resonate circuit?
     
  3. Runs212

    Thread Starter Member

    Mar 12, 2015
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    Gate voltages are above 10V. L = 24u and C is about 0.16uC at 80Khz
     
  4. ronv

    AAC Fanatic!

    Nov 12, 2008
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    It looks like you have a couple of problems:
    1- the gate resistor is pretty large so the FETs will turn on and off pretty slow for 80KHZ. You have about 100 ma of gate drive which will make the turn on/off time around 1.4 usec. To get this value from the data sheet you can look at NC. That is the time in ns. it will take to switch with one amp of gate current. Yours has an NC of 140 @ .1 amp = 1.4 usec.
    The second is as @cmartinez says you need some leisure time or dead time between the time you turn one FET on and the other off. As it is now they will both be on for a short period of time. While the time is short the current is very high. What you might try is to make the gate resistors 62 ohms and add a diode across each one with the cathode pointing towards the driver. This will make them turn off faster than they turn on and reduce the high current spike. It may not be enough, but should help. You may have to build the delay in logic before the driver.
     
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  5. ScottWang

    Moderator

    Aug 23, 2012
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    Your attached file size is too big for the circuit, I already re-size it from 811,430 KB to 26,623 Kb, that's a big difference and it is over ten times.
    Next time if you have any attachment, please upload a appropriate size, thanks.
     
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  6. ronv

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  7. Runs212

    Thread Starter Member

    Mar 12, 2015
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    Thanks for your inputs. I had increased the frequency to 125khz as a result, mosfets weren't heating up as fast as it was at 80khz. I would reduce the resistor to what you've recommended as well when I return to the lab.

    Another issue I'm having is I'm getting very low current. I took the current measurement by placing a 15R (5W) to the between the 0.1u capacitor and ground. I was reading 0.5A. I need at least 3A. My voltage is around 25V which is perfect. how I can I achieve that. Based on the data sheet, I can get as high as 55A for the drain current.
     
  8. Runs212

    Thread Starter Member

    Mar 12, 2015
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  9. Runs212

    Thread Starter Member

    Mar 12, 2015
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    Very useful information.
     
  10. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Here is the simulation with the diode and 62 ohms. Much better.
    The circuit is resonant at about 77KHZ so I'm guessing you should run it there. Is it a heater or a wireless charger??
    About 2.5 amps peak in the inductor.
     
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  11. ian field

    Distinguished Member

    Oct 27, 2012
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    At first glance - the 24u inductive load suggests pretty fast switching.

    The 120R gate-stopper resistors look too high - it will take a significant length of time to charge and discharge the gate capacitance.

    The MOSFETs are spending too much time dissipating in the linear region between fully on and fully off.
     
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  12. Runs212

    Thread Starter Member

    Mar 12, 2015
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    Thanks immensely ronv. This was tremendously helpful. I am building a wireless charger for your senior project. Among other things, I was having a significant drop in current when the phone was connected but after changing my resistor value and reducing my freq, "miraculously" :) , the phone charger increased it's charge by 1 after about a minute earning me an A- for my first semester final design project. I still need to optimize this system.

    Also, i have searched through LT Spice for the driver you've used in your simulation and couldn't find it. Could you point me in the right direction here? Thanks for your help ronv.
     
  13. Runs212

    Thread Starter Member

    Mar 12, 2015
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    This was taken into consideration by reducing my Rgate resistor to 62 and reducing my frequency to 77KHz which significantly improved the current. Thanks for your help.
     
  14. ronv

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    I think these are the right 2 files. It is a library file with several drivers.
     
  15. Runs212

    Thread Starter Member

    Mar 12, 2015
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    Hi Ronv, I'm working on this circuit again. I'm curious to know how you arrived at 77Khz for the resonance circuit. Using the f = 1/(2PiSqr(LC)) doesn't quite place the resonance at 77Khz. Interestingly, on the receiver circuit, when i used the 24uH on the transmitter ckt (what i have above) and 6.3uH coil on the receiver side, the system didn't work. When i reversed coils with everything else remainig unchanged, the system conducted current. I'm unsure why that's the case considering resonance frequency should ensure maximum output.

    Also, could you attach the LT spice file you designed above if it's stilll available? I'm
     
  16. ronv

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  17. Runs212

    Thread Starter Member

    Mar 12, 2015
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    Thanks Ronv. Everything seems to work ok except the current. I've getting over 40v - 52V peak-to-peak on the receiver circuit but that is regulated to 5V using a dc regulator. However, the current is very low. Often below 0.1A which isn't sufficient to charge a phone which requires at 5W.
    I noticed on my Vdd (power source), It's rating is +/_ 20V, 0.5A. I'm using around 12V for my circuit. is that supposed to me the maximum current it sources and could that explain why i'm getting very low current? If so, what suggesting do you have get through this?







     
  18. ronv

    AAC Fanatic!

    Nov 12, 2008
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    At resonance the impedance of the LC is very low so the current is quite high - So yes you need a larger supply. You might also want to add some resistance (1/2 - 1 ohm) in series with the LC at least until you can see how much current it does draw.
    Add some load to the output of the receiver until the voltage drops, then measure the current.

    PS.
    What does your receiver circuit look like?
     
    Last edited: Sep 16, 2015
  19. Runs212

    Thread Starter Member

    Mar 12, 2015
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    Revisiting this, I have been having serious issues with my circuit. I realized that the gate current should be taken into consideration in my system here. Based on the above topology, I have 25Ohms resistors at the output of my drivers. The high side measures 104mA while the low side measures around 52mA at 15V Vcc. The frequency is 125KHz.

    Help me understand this:
    For the MOSFET, it's Qg = 140nC T(on)= 100ns. Hence, the Gate current required = Qd/Td = 1.4A.

    Based on the above info, am i right to say that the mosfets wouldn't sufficient charge/turn based on my measured outputs? Hence, would replacing my mosfet be necessary?

    I have been beating this circuit for weeks and i'm not making any progress in understanding why it's not supplying a high current at the load.
     
  20. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Like ronv asked, how does your receiver look like, and what arrangement are the coils? You just might not have enough coupling to get the power through.
     
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